1. Field of the Invention
The present invention relates to an image forming apparatus, a control method therefor and a program for implementing the control method, and more particularly relates to an image forming apparatus that electrophotographically forms an image on a recording medium, by primarily transferring a toner image formed on a photosensitive member onto an intermediate transfer member, and then secondarily transferring the toner image on the intermediate transfer member onto the recording medium, such as a copying machine, a multifunction apparatus, and a printer, as well as a control method for the image forming apparatus and a program for implementing the control method.
2. Description of the Related Art
There has been known an image forming apparatus that electrophotographically forms an image, such as a copying machine, a multifunction apparatus, and a printer, in which a toner image formed on a photosensitive member is once primarily transferred onto an intermediate transfer member, the toner image is then secondarily transferred onto a recording medium such as a recording sheet or an OHP sheet, and the toner image on the recording medium is fixed, thereby forming an image. As the intermediate transfer member used in the above described transfer, a drum-shaped intermediate transfer member and a belt-shaped intermediate transfer member are actually used. The intermediate transfer belt method using the belt-shaped intermediate transfer member is currently attracting attention due to it being advantageous in saving installation space in an image forming apparatus since the miniaturization of image forming apparatuses has been desired these days.
When a full-color image is formed by an image forming apparatus that carries out the transfer using the intermediate transfer belt, since it is difficult to form overlapped toner images on the photosensitive member, toner images of three colors of yellow, cyan, and magenta or those of four colors including black in addition to these three colors are sequentially primarily transferred from the photosensitive member onto the intermediate transfer belt, and the toner images of the full color overlapped on the intermediate transfer belt are secondarily transferred onto a recording medium at once, thereby forming a full-color image.
To achieve a good image quality of the full-color image obtained by the above described process, it is necessary to accurately align the multi-color toner images to be overlapped on the intermediate transfer belt. Specifically, if the toner images in three colors or four colors are slightly displaced from the position in which they are to be overlapped, the resulting image has a color completely different from that of the original image formed on a medium such as an original, which necessitates carrying out the accurate alignment.
Conventionally, to accurately align multi-color toner images on the intermediate transfer belt, a reference mark serving as a reference of the image formation timing is provided at a predetermined position on the intermediate transfer belt, the reference mark is detected by an optical sensor or the like provided at a predetermined position on a conveying path for the intermediate transfer belt, and the image forming process is started in predetermined timing after the detection of the reference mark so that the multi-color toner images are primarily transferred and overlapped at a given position on the intermediate transfer belt. In addition, other improved techniques have been proposed for more accurate alignment of multi-color toner images (for example, Japanese Laid-Open Patent Publications (Kokai) No. H7-92763 and No. H7-281536).
However, if the image formation is carried out successively using these conventional methods, a defect may occur in the image due to degradation of the intermediate transfer belt. Specifically, according to these methods, since the toner images are always overlapped at a certain area on the intermediate transfer belt, there occurs such a phenomenon that an aging change of the state of a conducting agent inside the intermediate transfer belt causes a decrease in the resistance value of that area on the intermediate transfer belt. Such decrease in the resistance value of the specific area on the intermediate transfer belt causes a difference in primary and second transferability between the area having the decreased resistance and the other areas, and an image defect such as a void becomes remarkable when a large halftone image is formed across the area having the decreased resistance value and another area.
To solve this problem, there has been proposed a technique that a plurality of reference marks are provided on the intermediate transfer belt, any one of these reference marks is detected by a photo sensor, the timing of exposure on the photosensitive members is controlled to predetermined timing so as to accurately align multi-color toner images formed, and at the same time, primarily transfer the toner images at different positions on the intermediate transfer belt (for example, Japanese Laid-Open Patent Publication (Kokai) No. H8-146698).
When the timing of the image forming process is controlled based on the plurality of reference marks provided on the intermediate transfer belt as above, an identification mark should be added to each reference mark for identification, and the control should be carried out while the identification mark is identified using a sensor. Specifically, for example, if a yellow toner image is transferred onto the intermediate transfer belt with reference to a reference mark “a” provided at a predetermined position on the intermediate transfer belt, the reference mark “a” must be also used as a reference when the next toner image such as a cyan toner image is transferred onto the intermediate transfer belt to overlap the next toner image on the yellow toner image. If another reference mark “b” is used as a reference, a color misalignment occurs.
However, there is such a case where the sensor cannot identify the identification mark added to the reference mark on the intermediate transfer belt which rotates in synchronism with the speed of image formation on the recording medium. Particularly, recently, high speed image formation has been required, so that it is difficult for the sensor to accurately read the identification marks on the intermediate transfer belt which rotates at a high speed for such high speed image formation. Although this problem can be solved by using a high performance sensor which can accurately read the identification marks even if the intermediate transfer belt is rotating at a high speed, such a sensor is disadvantageous in terms of cost. Apart from this problem, there is a problem that the identification marks disappear when the surface of the intermediate transfer belt is cleaned using a cleaning blade, and consequently the sensor cannot read the identification marks on the intermediate transfer belt. In these cases, the proper timing control cannot be carried out, and as a result, a color misalignment may occur.
Further, if the timing of the image forming process is controlled based on the plurality of reference marks provided on the intermediate transfer belt as described above, after preparation for (toner) image formation for a first color has been completed, a first reference mark is detected and then image formation is started. As a result, at least a wait time period from the completion of preparation for the image formation to the detection of the first reference mark is added to a FCOT (first copy out time) for the full-color image formation.
Therefore, a method for actively reducing the above described wait time has recently been studied. According to this method, the circumferential length in the circumferential direction (rotational direction) of the intermediate transfer member is detected and stored in a RAM or the like in advance. After the preparation for image formation is completed, image formation start signals are generated in arbitrary timing according to a program. Specifically, an image formation start signal for a first color is generated in arbitrary timing, and then a next image formation start signal for a next color is generated upon the lapse of a one-turn time period required for the intermediate transfer member to make one turn, which is calculated from the stored circumferential length and the rotational speed of the intermediate transfer member. As a result, the wait time until the detection of the first reference mark can be eliminated, providing an advantage of reduction of the FCOT for the full-color image formation compared with the method of starting the image formation based on the reference marks (for example, Japanese Laid-Open Patent Publication (Kokai) No. H10-20614).
Further, in the case where the image formation start signal is generated using the one-turn time period calculated in advance as described above, when a plurality of full-color images are successively output, there has been the problem that various mechanical shocks or mechanical load fluctuations occur due to contacting and separation of the cleaning blade with and from the intermediate transfer member, that is, a mechanical shock caused by the separation of the cleaning blade from the intermediate transfer member when a toner image is formed on the intermediate transfer member for a first color of a first recording sheet; a mechanical shock caused by contacting of a secondary transfer roller with the recording sheet when a color toner image is secondarily transferred on a recording sheet after a tone image of a fourth color is overlapped on the intermediate transfer member; a mechanical shock caused by contacting of the cleaning blade with the intermediate transfer member for cleaning the same; and other mechanical load fluctuations caused by contacting and separation of the cleaning blade with and from the intermediate transfer member. These mechanical load fluctuations cause variations in the rotational speed of the intermediate transfer member such that the one-turn time period varies between the respective colors. This results in a color misalignment between the first color and second and subsequent colors in the color overlapping process.